1、Standard Method of Test for Determining the Fracture Energy of Asphalt Mixtures Using the Semicircular Bend Geometry (SCB) AASHTO Designation: TP 105-13 (2015)1American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-2d TP
2、105-1 AASHTO Standard Method of Test for Determining the Fracture Energy of Asphalt Mixtures Using the Semicircular Bend Geometry (SCB) AASHTO Designation: TP 105-13 (2015)11. SCOPE 1.1. This test method covers the determination of the fracture energy (Gf) of asphalt mixtures by means of the semicir
3、cular bend geometry (SCB). The method also includes procedures for calculating fracture toughness (KIC) and stiffness (S). The SCB specimen is a half disc with a notch (its length expressed in meters) that makes an angle with the vertical axis of the disc. The SCB test can be used to determine mode
4、I and mixed mode I and II stress intensity factors (Lim et al., 1993). In this standard, only mode I fracture toughness is addressed ( is equal to zero). 1.2. The procedures in this standard provide parameters that describe the fracture resistance of asphalt mixtures at low temperatures. These param
5、eters are used in the new low-temperature module of the Mechanistic Empirical Pavement Design Guide. 1.3. These procedures apply to test specimens having a maximum aggregate size of 19 mm or less. Specimens shall be 150 9 mm in diameter and 24.7 mm 2 mm thick. These procedures are valid at temperatu
6、res below the performance grade (PG) lower limit of the asphalt binder used to prepare the asphalt mixture plus 22C. 1.4. This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety concerns associated with its use. It is the
7、responsibility of the user of this procedure to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to use. 2. REFERENCED DOCUMENTS 2.1. AASHTO Standards: M 320, Performance-Graded Asphalt Binder T 166, Bulk Specific Gravity (Gmb) of C
8、ompacted Hot Mix Asphalt (HMA) Using Saturated Surface-Dry Specimens T 269, Percent Air Voids in Compacted Dense and Open Asphalt Mixtures T 312, Preparing and Determining the Density of Asphalt Mixture Specimens by Means of the Superpave Gyratory Compactor T 322, Determining the Creep Compliance an
9、d Strength of Hot Mix Asphalt (HMA) Using the Indirect Tensile Test Device 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d TP 105-2 AASHTO 2.2. ASTM Standards: D8, Standard Terminology Relating to
10、Materials for Roads and Pavements D3549/D3549M, Standard Test Method for Thickness or Height of Compacted Bituminous Paving Mixture Specimens D4123, Standard Test Method for Indirect Tension Test for Resilient Modulus of Bituminous Mixtures (Withdrawn 2003) D5045, Standard Test Methods for Plane-Str
11、ain Fracture Toughness and Strain Energy Release Rate of Plastic Materials D5361/D5361M, Standard Practice for Sampling Compacted Bituminous Mixtures for Laboratory Testing E399, Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness KICof Metallic Materials 3. TERMINOLOGY 3.1. Defi
12、nitions: 3.1.1. crack mouthportion of the notch that is on the flat bottom surface of the specimen, that is, opposite the notch tip. 3.1.2. crack mouth opening displacement (CMOD)relative displacement of the crack mouth. 3.1.3. load line displacement (LLD)the displacement measured in the direction o
13、f the load application. 3.1.4. linear variable differential transformer (LVDT)sensor device for measuring linear displacement. 3.1.5. semicircular bend (SCB) geometrya geometry that utilizes a semicircular specimen. 3.1.6. fracture energy, Gfthe energy required to create a unit surface area of a cra
14、ck. 3.1.7. stiffness, Sthe slope of the linear part of the ascending load-load line displacement curve. 3.1.8. linear elastic fracture mechanics (LEFM)a method of fracture analysis for determining the stress required to induce fracture instability in a structure containing a crack-like flaw of known
15、 size and shape. 3.1.9. mode I stress intensity factor, KIthe parameter used to characterize the mode I stress field in the vicinity of the crack tip in the LEFM analysis. 3.1.10. mode I critical stress intensity factor, KICstress intensity factor corresponding to the initiation of the crack, also r
16、eferred to as fracture toughness. 4. SUMMARY OF TEST METHOD 4.1. A semicircular asphalt mixture specimen is positioned with the flat side on two rollers that are covered with a friction-reducing material. A load is applied along the vertical diameter of the specimen and the load and load line displa
17、cement are measured during the entire duration of the test. The load is applied such that a constant CMOD of 0.0005 mm/s is obtained and maintained for the duration of the test to ensure stable crack growth conditions. 2015 by the American Association of State Highway and Transportation Officials.Al
18、l rights reserved. Duplication is a violation of applicable law.TS-2d TP 105-3 AASHTO 4.2. Fracture energy (Gf), stiffness (S), and fracture toughness (KIC) are calculated from the load and load line displacement results. 5. SIGNIFICANCE AND USE 5.1. The SCB test is used to determine the low-tempera
19、ture fracture energy and fracture toughness of asphalt mixtures. These parameters describe the fracture resistance of asphalt mixtures. 5.2. Fracture energy can be used as an index parameter to identify mixtures with increased fracture resistance. It also represents the main parameter used in more c
20、omplex analyses based on a fictitious crack (cohesive zone) model. 5.3. Fracture toughness obtained with this test method can be used as an index parameter to identify mixtures with increased fracture resistance. 5.4. Stiffness obtained with this test method can be related to the elastic modulus of
21、asphalt mixtures at low temperatures. 5.5. The specimens can be easily cut from Superpave gyratorycompacted cylinders and from field cores with a diameter of 150 mm. 6. APPARATUS 6.1. Semicircular Bend (SCB) Test SystemA semicircular bend (SCB) test system consisting of a closed-loop axial loading d
22、evice, a load measuring device, a bend test fixture, specimen deformation measurement devices, an environmental chamber, and a control and data acquisition system (see Figure 1). 6.1.1. Axial Loading DeviceThe loading device shall be capable of delivering a minimum load of 10 kN in compression with
23、a resolution of 20 N or better. The load apparatus shall be capable of maintaining a constant crack mouth opening displacement within 1 percent of the target value throughout the test. The loading head shall be similar to the one described for the bend test fixture in ASTM E399 (see Figure 1). 2015
24、by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d TP 105-4 AASHTO Figure 1SCB Loading Setup for Front and Back Sides of Test Specimen 6.1.2. Load Measuring DeviceThe load measuring device shall consist of
25、 an electronic load cell, designed for placement between the loading platen and piston, with a minimum capacity of 10 kN and a sensitivity of 10 N or better. 6.1.3. Bend Test FixtureThe test fixture is composed of a steel base plate, two L-shaped roller support steel blocks, two steel rollers, and t
26、wo U-shaped frames (see Figure 2). The loading fixture is designed to minimize frictional effects through the use of rollers (as suggested by ASTM E399). The surface of the rollers is covered with polytetrafluoroethylene (PTFE) strips to further reduce friction. The initial roller position is mainta
27、ined by soft springs and backstops that establish the test span dimension. The support rollers are allowed to rotate out away from the backstops during the test but will remain in contact with the sample. The roller support blocks are secured to a 12.7-mm thick base plate with a 9.5-mm diameter dowe
28、l hole for alignment with the actuator center. To obtain an accurate measure of the load-line displacement (LLD), two U-shaped reference frames are secured to the L-shaped roller support blocks, one on each side of the blocks. A steel gauge point (see Section 6.1.5) is permanently attached to the ce
29、nter of the upper arm of each of the reference frames. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d TP 105-5 AASHTO Figure 2SCB Test Fixture 6.1.4. Specimen Deformation Measuring DevicesThe spec
30、imen deformation measurement devices shall consist of a CMOD gauge and two LLD gauges, with a range of at least 1 mm and a resolution of 0.0005 mm or better. The CMOD gauge is attached to the two gauge points glued to the bottom of the SCB specimen via knife edges (see Figures 3 and 4). The two LLD
31、gauges are attached to the gauge points glued to the specimens front and back sides and the corresponding gauge points on the two U-shaped reference frames. 6.1.5. Gauge PointsTwo steel gauge points having a diameter of 8 mm and a height of 6 mm and two knives are required per specimen (see Figures
32、3 and 4). 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d TP 105-6 AASHTO Figure 3Gauge Points Locations Figure 4CMOD Gauge Knife Edges 6.1.6. Mounting TemplateA mounting template for placing and m
33、ounting the two steel gauge points on the SCB specimen is shown in Figure 5. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d TP 105-7 AASHTO Figure 5Gauge Points Template 6.1.7. Environmental Chamb
34、erThe environmental chamber should be equipped with a temperature conditioner and controls capable of generating a test temperature between 40C and 0C inside the chamber and maintaining the desired test temperature to within 0.5C. The internal dimensions of the environmental chamber should be capabl
35、e of holding a minimum of three test specimens for a period of 2 0.5 h prior to testing. 6.1.8. Control and Data Acquisition SystemSpecimen behavior during the semicircular bend test is evaluated from time records of applied load, CMOD, and LLD. The applied load is controlled via the closed loop by
36、the CMOD rate being kept constant during the test. 7. HAZARDS 7.1. Observe standard laboratory safety precautions when preparing and testing HMA specimens. 8. STANDARDIZATION 8.1. The testing system should be calibrated prior to initial use and at least once a year thereafter. 8.1.1. Verify the capa
37、bility of the environmental chamber to maintain the required temperature within the specified accuracy. 8.1.2. Verify the calibration of all measurement components (such as load cells and LVDTs) of the testing system. 8.1.3. If any of the verifications yield data that does not comply with the accura
38、cy specified, correct the problem prior to proceeding with testing. Appropriate action may include maintenance of system components, calibration of system components (using an independent calibration agency, service by the manufacturer, or in-house resources), or replacement of the system components
39、. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d TP 105-8 AASHTO 9. SAMPLING 9.1. Laboratory-Molded SpecimenPrepare three replicate laboratory-molded specimens, as a minimum for each test temperat
40、ure, in accordance with T 312. 9.2. Roadway SpecimenObtain roadway specimens from the pavement in accordance with ASTM D5361/D5361M. Prepare cores with smooth and parallel surfaces that conform to the height and diameter requirements specified in Section 10.2. Prepare three replicate cores for each
41、test temperature. 10. SPECIMEN PREPARATION AND PRELIMINARY DETERMINATIONS 10.1. Specimen SizeFor mixtures with maximum aggregate size of 19 mm or less, prepare specimens with a thickness of 24.7 2 mm and a diameter of 150 9 mm (see Figure 3). 10.2. SGC SpecimensPrepare three SGC specimens according
42、to T 312. From the center of each 115 5 mm-tall specimen, obtain a cylindrical slice that is 24.7 2 mm thick (see Figure 6). Cut the slice in two identical “halves” and then cut a notch along the axis of symmetry of each half that is 15 0.5 mm in length and no wider than 1.5 mm (see Figures 3 and 6)
43、. Use one half from each cylinder for testing at one test temperature (T1) and the other half for testing at the second test temperature (T2). If more replicates or test temperatures are necessary, cut additional 24.7 2 mm-thick slices from the SGC cylinder, located as close to the middle slice as p
44、ossible. Figure 6Sample Preparation 10.3. Field CoresField cores can also be used to fabricate the specimens. The target thickness for specimens obtained from field cores should be 24.7 2 mm. The top and bottom of the core shall be cut to ensure that the test specimen has parallel faces. If multiple
45、 slices are cut from taller cores, the lift thickness shall be considered to obtain representative samples. Note 1A typical laboratory saw for mixture specimen preparation can be used to obtain cylindrical slices with smooth parallel surfaces. Diamond-impregnated cutting faces and water-cooling are
46、recommended to minimize damage to the specimen. When cutting the SCB specimens, it is recommended not to push the two halves against each other because it may create an uneven base surface of the test specimen that will significantly affect the results. 10.4. Determining Specimen DimensionsMeasure a
47、nd record the diameter and thickness of each specimen in accordance with ASTM D3549/D3549M, and determine individual measurements to the nearest 1 mm. Measure the notch length on both faces of the specimen and record the average value to the nearest 0.5 mm. 10.5. Determining the Bulk Specific Gravit
48、yDetermine the bulk specific gravity directly on the 115 5 mm-tall SGC specimen in accordance with T 312. 10.6. Specimen DryingIf specimens were immersed directly into water, after determining the bulk specific gravity, allow each specimen to dry at room temperature to a constant mass. 2015 by the A
49、merican Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d TP 105-9 AASHTO 10.7. Mounting Deformation Measuring DevicesSuperglue the four gauge points on the specimen as shown in Figure 6. A template, similar to the one in Figure 5, can be used for this purpose. 11. TEST PROCEDURE 11.1. ConditioningThe specimens shall be placed in a temperature controlled chamber at the desired test temperature for 2 0.5 h. T
